1. Field of the Invention
The present invention relates to a displaying device for displaying video signals, and more particularly, to a multi-frequency displaying device for displaying both TV (television) and PC (personal computer) video signals.
2. Description of the Prior Art
TV monitors shared to display both TV and PC video signals are commonly seen in many home electronics or commercial advertisement applications because big TV monitors are much cheaper than comparable computer monitors. But since TV video signals and PC video signals have various horizontal displaying speeds, they must be processed first before displaying over a displaying screen of a TV monitor.
Please refer to FIG.1. FIG.1 shows a block diagram of a prior art displaying device 10 which is a TV monitor for displaying both TV video signals (NTSC system, 15.75 kHz horizontal frequency) and PC video signals (VGA--video graphic array, 31.47 kHz horizontal frequency) in one horizontal frequency (31.47 kHz) over a CRT (cathode ray tube) 18. The PC video signals comprises PC RGB signals from three signal lines and PC sync signals (H/V) from two signal lines of a cable connected to a personal computer (not shown).
The device 10 comprises a front end circuit 11 for converting TV video signals over port 111 into TV Y/R-Y/B-Y video signals over port 112 and TV sync signals over port 113, a dual scan circuit 12 for doubling each TV Y/R-Y/B-Y video signals, a video signal processing circuit 13 for converting the TV Y/R-Y/B-Y video signals from the dual scan circuit 12 into TV RGB signals which are output over port 131, a timing generator 16 for generating clock signals for the dual scan circuit 12 and also doubling the frequency of the TV sync signals from 15.75 kHz into 31.47 kHz, a sync circuit 17 for converting the TV sync signals from the timing generator 16 or PC sync signals into vertical and horizontal deflection signals over port 171 and 172 according to a switch 15 (connection not shown), and a display control circuit 14 for displaying the TV RGB signals or PC RGB signals synchronized with the vertical and horizontal deflection signals from the sync circuit 17 according to the switch 15. The dual scan circuit 12 comprises an A/D (analog to digital) converter 121 for digitizing the TV Y/R-Y/B-Y video signals, a dual scan processing unit 122 for processing each digitized video signal to achieve the dual scan effect, and a D/A (digital to analog) converter for converting the output from the dual scan processing circuit 122 into analog output.
The D/A and A/D conversion processes performed by the dual scan circuit 12 cause certain losses in the video images contained in the output TV Y/R-Y/B-Y video signals when compared with the input TV Y/R-Y/B-Y video signals and thus reduces the image quality displayed in the CRT 18 of the displaying device 10. Besides, the dual scan circuit 12 coupled with the timing generator 16 for doubling TV video signals are quite complex when compared with a conventional TV.